Adrenal gland samples from 34 individuals 0-68 yr of age were dissected into four layers of equal thickness parallel to the capsule and the zonal boundaries of the cortex, and were analyzed by RIA for their concentrations of 17OH-progesterone, 11-desoxycortisol, cortisol, progesterone, corticosterone, aldosterone, 17OH-pregnenolone, dehydroepiandrosterone, androstenedione, and testosterone, with DNA content to correct for variations in sample size. Intraadrenal steroid concentrations were 10-1000 times higher than reported serum levels, and varied significantly with age, adrenal weight, and cortical thickness. The concentrations of cortisol,11-desoxycortisol, corticosterone, androstenedione, and testosterone increased with age. Levels of 17OH-pregnenolone, dehydroepiandrosterone, and 17OH-progesterone decreased during infancy in parallel with involution of the adrenal, and then rose again in late childhood and puberty. The concentrations of all steroids, except aldosterone, increased significantly from the outer to the inner layers of cortex. Comparisons of product-substrate ratios suggested that changes in adrenal androgen secretion are induced by shifts in the relative activities of key branch-point steroidogenic enzymes, notably 3 beta-hydroxysteroid dehydrogenase-isomerase and 17,20-desmolase. Since intraadrenal steroid concentrations are in the range of the Michaelis-Menten constant (Km) for these enzymes (10(-6) M) and can be shown in vitro at this concentration to influence relative enzyme activities, these data support the hypothesis that zonal and developmental changes in adrenal androgen secretion relative to that of cortisol are induced by changes in the steroidal micro-environment which in turn are imposed by adrenal growth and the centripetal blood flow through the gland.